Current methods of genome engineering typically introduce one DNA construct per cell, generally at low efficiency (around 0.1%). Sometimes a large collection of constructs is introduced into a large number of cells simultaneously, in a single tube, to produce a clone ‘library’, but the intention is still typically to have one DNA type per cell. To eliminate the many surviving unwanted cells lacking any new DNA, typically a selection and/or screen is performed at each step of a multi-step construction. It is rare to complete a genome engineering construct with more than a dozen steps.